An important challenge facing society today is securing a sustainable fuel supply for the transportation sector. The transportation sector currently relies almost exclusively on liquid hydrocarbons as its energy source. The high energy density of gasoline, 8.88 kWh/liter, far exceeds that of proposed replacements H2, 1.32 kWh/liter at 680 atm [1], or batteries, 175 Wh/kg [2]. Furthermore, the liquid hydrocarbon fuel distribution infrastructure is efficient and already in place.
Production of liquid fuels from biomass can help solve the problem of CO2 emission from the transportation sector because CO2 released from vehicle exhaust is captured during biomass growth. Currently, there are two major routes for conversion of biomass to liquid fuels: biological and thermo-chemical. In the biological process, fermentation of easily fermentable plant products, such as for example sugar, sucrose, dextrose and etc., to alcohols is achieved. These easily fermentable plant products can be extracted from corn kernels, sugar cane and etc. Major disadvantage of this pathway is that only a fraction of the total carbon in biomass is converted to the final desired liquid hydrocarbon fuel. Hill et al. calculate that conversion of all corn produced in USA to ethanol can meet 12% of entire US demand for gasoline which reduces to 2.4% after accounting for fossil fuel input [3].
Thermo-chemical routes include gasification of biomass to obtain synthesis gas (syngas), a mixture of carbon monoxide (CO) and hydrogen (H2), and conversion of the syngas gas to liquid fuels using the Fischer-Tropsch (FT) process. A quick estimate can be made for the land area requirement to support the current oil consumption of 13.8 million barrels per day by the US transportation sector. For this purpose, biomass gasification data available from the Department of Energy (DOE) H2A analysis can be used. Assuming conversion of syngas to diesel to be 100% efficient by the FT process, then the land area required for a biomass growth rate of 15 ton/ha/yr and a gasifier efficiency of 74% is estimated to be about 2.67 million square km which represents 29% of the total US land area, thus suggesting that large scale production of liquid fuels from such a biomass conversion process is impractical.
Accordingly, further improvements and enhancements in the production of liquid fuels from biomass are desirable.